High permeability superabsorbent polymer compositions

ABSTRACT

The invention relates to absorptive, crosslinked polymeric composition that are based on partly neutralized, monoethylenically unsaturated monomer carrying acid groups wherein the absorptive crosslinked polymer may be coated with a polymeric coating, and have improved properties, in particular in respect of their capacity for transportation of liquids in the swollen state, and which have a high capacity and a high gel bed permeability.

This application is a divisional application of U.S. patent applicationSer. No. 13/524,232 filed on Jun. 15, 2012, now U.S. Pat. No. 8,519,041,which is a continuation application of U.S. patent application Ser. No.11/690,611 filed on Mar. 23, 2007, now U.S. Pat. No. 8,236,884, thedisclosures of which is expressly incorporated herein by reference.

BACKGROUND

A superabsorbent material in general refers to a water-swellable,water-insoluble, material capable of absorbing at least about 10 timesits weight, and up to about 30 times or more its weight in an aqueoussolution containing 0.9 weight percent sodium chloride solution inwater. The present invention relates to superabsorbent polymercompositions, which absorb water, aqueous liquids, and blood.

A superabsorbent polymer is a crosslinked partially neutralized polymerthat is capable of absorbing large amounts of aqueous liquids and bodyfluids, such as urine or blood, with swelling and the formation ofhydrogels, and of retaining them under a certain pressure in accordancewith the general definition of superabsorbent material. Superabsorbentpolymer compositions may include post-treatment of the superabsorbentpolymer such as surface crosslinking, surface treatment, and othertreatment. Superabsorbent polymer particles are particles ofsuperabsorbent polymers or superabsorbent polymer compositions. Theacronym SAP may be used in place of superabsorbent polymer,superabsorbent polymer composition, and particles herein. Acomprehensive survey of superabsorbent polymers, and their use andmanufacture, is given in F. L. Buchholz and A. T. Graham (editors) in“Modern Superabsorbent Polymer Technology,” Wiley-VCH, New York, 1998.

Commercially available superabsorbent polymer compositions includecrosslinked polyacrylic acids or crosslinked starch-acrylic acid graftpolymers, in which some of the carboxyl groups are neutralized withsodium hydroxide solution or potassium hydroxide solution. A primary useof superabsorbent polymer compositions is in sanitary articles, such asbabies' diapers, incontinence products, or sanitary towels. For fit,comfort, and aesthetic reasons, and from environmental aspects, there isan increasing trend to make sanitary articles smaller and thinner. Thisis being accomplished by reducing the content of the high volume flufffiber in these articles. To ensure a constant total retention capacityof body fluids in the sanitary articles, more superabsorbent polymercontent is being used in these sanitary articles.

Permeability is a measure of the effective connectedness of a porousstructure, be it a mat of fiber, or a slab of foam or, in this case,crosslinked polymers, and may be specified in terms of the voidfraction, and extent of connectedness of the superabsorbent polymercomposition. Gel permeability is a property of the mass of particles asa whole and is related to particle size distribution, particle shape,and the connectedness of the open pores between the particles, shearmodulus, and surface modification of the swollen gel. In practicalterms, the gel permeability of the superabsorbent polymer composition isa measure of how rapidly liquid flows through the mass of swollenparticles. Low gel permeability indicates that liquid cannot flowreadily through the superabsorbent polymer composition, which isgenerally referred to as gel blocking, and that any forced flow ofliquid (such as a second application of urine during use of the diaper)must take an alternate path (e.g., diaper leakage).

One method to increase permeabilities in extremely thin diapers with lowfiber content is to increase the amount of crosslinking of thesuperabsorbent polymer composition. However, the absorption andretention values of the superabsorbent polymer compositions are reducedto undesirably low levels when the crosslinking of the superabsorbentpolymer is increased.

It is an object of the present invention to provide superabsorbentpolymer compositions possessing improved application propertiesincluding a high absorption capacity to retain fluids under no load,high absorption capacities to retain fluid under pressure, and improvedgel bed permeability.

SUMMARY

An embodiment of the present invention comprises at least a highpermeability superabsorbent polymer composition comprisingsuperabsorbent polymer particles surface-treated with from about 0.01%to about 2% by weight of an inorganic metal compound, based on the drysuperabsorbent polymer composition, wherein the superabsorbent polymercomposition exhibits a Centrifuge Retention Capacity of at least about30 g/g and a free swell gel bed permeability of at least 10 Darcy asmeasured by the Free Swell Gel Bed Permeability Test.

In addition, another embodiment of the present invention comprises aprocess of treating superabsorbent polymer particles withfinely-divided, water-insoluble inorganic metal salt comprising thesteps of a) providing superabsorbent polymer particles; b) preparing afirst solution of a first inorganic metal salt; c) adding to and mixingwith the first solution of b) a second solution of a second inorganicmetal salt, wherein the first solution and second solution react onmixing to precipitate a third water-insoluble metal salt to form awater-insoluble metal salt slurry; d) optionally oxidizing the metal ofthe water-insoluble metal salt slurry to a higher valence state; and e)applying the water-insoluble metal salt slurry to a superabsorbentpolymer particles without isolation and drying of the water-insolublemetal salt slurry.

In addition, another embodiment of the present invention comprises aprocess of treating superabsorbent polymer particles withfinely-divided, water-insoluble inorganic metal salt comprising thesteps of a) providing superabsorbent polymer particles; b) preparing afirst solution of a first inorganic metal salt; c) preparing a secondsolution of a second inorganic metal salt; d) applying the firstsolution and second solution to the superabsorbent polymer particles toform a water-insoluble inorganic metal salt precipitate directly on orin the vicinity of the surface of the superabsorbent polymer particles.

Numerous other features and advantages of the present invention willappear from the following description. In the description, reference ismade to exemplary embodiments of the invention. Such embodiments do notrepresent the full scope of the invention. Reference should therefore bemade to the claims herein for interpreting the full scope of theinvention. In the interest of brevity and conciseness, any ranges ofvalues set forth in this specification contemplate all values within therange and are to be construed as support for claims reciting anysub-ranges having endpoints which are real number values within thespecified range in question. By way of a hypothetical illustrativeexample, a disclosure in this specification of a range of from 1 to 5shall be considered to support claims to any of the following ranges:1-5; 1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5.

In addition, the present invention is directed to absorbent compositionsor sanitary articles that may contain superabsorbent polymercompositions of the present invention.

FIGURES

The foregoing and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a side view of the test apparatus employed for the Free SwellGel Bed Permeability Test;

FIG. 2 is a cross-sectional side view of a cylinder/cup assemblyemployed in the Free Swell Gel Bed Permeability Test apparatus shown inFIG. 1;

FIG. 3 is a top view of a plunger employed in the Free Swell Gel BedPermeability Test apparatus shown in FIG. 1; and

FIG. 4 is a side view of the test apparatus employed for the AbsorbencyUnder Load Test.

DEFINITIONS

It should be noted that, when employed in the present disclosure, theterms “comprises,” “comprising,” and other derivatives from the rootterm “comprise” are intended to be open-ended terms that specify thepresence of any stated features, elements, integers, steps, orcomponents, and are not intended to preclude the presence or addition ofone or more other features, elements, integers, steps, components, orgroups thereof.

The term “absorbent article” generally refers to devices that can absorband contain fluids. For example, personal care absorbent articles referto devices that are placed against or near the skin to absorb andcontain the various fluids discharged from the body.

The term “disposable” is used herein to describe absorbent articles thatare not intended to be laundered or otherwise restored or reused as anabsorbent article after a single use. Examples of such disposableabsorbent articles include, but are not limited to, personal careabsorbent articles, health/medical absorbent articles, andhousehold/industrial absorbent articles.

The term “crosslinked” used in reference to the superabsorbent polymerrefers to any means for effectively rendering normally water-solublematerials substantially water-insoluble but swellable. Such acrosslinking means can include, for example, physical entanglement,crystalline domains, covalent bonds, ionic complexes and associations,hydrophilic associations such as hydrogen bonding, hydrophobicassociations, or Van der Waals forces.

The term “Darcy” is a CGS unit of permeability. One Darcy is thepermeability of a solid through which one cubic centimeter of fluid,having a viscosity of one centipoise, will flow in one second through asection one centimeter thick and one square centimeter in cross-section,if the pressure difference between the two sides of the solid is oneatmosphere. It turns out that permeability has the same units as area;since there is no SI unit of permeability, square meters are used. OneDarcy is equal to about 0.98692×10⁻¹² m² or about 0.98692×10⁻⁸ cm².

The term “dry superabsorbent polymer composition” generally refers tothe superabsorbent polymer composition having less than about 10%moisture.

The term “mass median particle size” of a given sample of particles ofsuperabsorbent polymer composition is defined as the particle size,which divides the sample in half on a mass basis, i.e., half of thesample by weight has a particle size greater than the mass medianparticle size, and half of the sample by mass has a particle size lessthan the mass median particle size. Thus, for example, the mass medianparticle size of a sample of superabsorbent polymer compositionparticles is 2 microns if one-half of the sample by weight is measuredas more than 2 microns.

The terms “particle,” “particulate,” and the like, when used with theterm “superabsorbent polymer,” refer to the form of discrete units. Theunits can comprise flakes, fibers, agglomerates, granules, powders,spheres, pulverized materials, or the like, as well as combinationsthereof. The particles can have any desired shape: for example, cubic,rod-like, polyhedral, spherical or semi-spherical, rounded orsemi-rounded, angular, irregular, et cetera. Shapes having a high aspectratio, like needles, flakes, and fibers, are also contemplated forinclusion herein. The terms “particle” or “particulate” may also includean agglomeration comprising more than one individual particle,particulate, or the like. Additionally, a particle, particulate, or anydesired agglomeration thereof may be composed of more than one type ofmaterial.

The term “polymer” includes, but is not limited to, homopolymers,copolymers, for example, block, graft, random, and alternatingcopolymers, terpolymers, etc., and blends and modifications thereof.Furthermore, unless otherwise specifically limited, the term “polymer”shall include all possible configurational isomers of the material.These configurations include, but are not limited to isotactic,syndiotactic, and atactic symmetries.

The term “polyolefin” as used herein generally includes, but is notlimited to, materials such as polyethylene, polypropylene,polyisobutylene, polystyrene, ethylene vinyl acetate copolymer, and thelike, the homopolymers, copolymers, terpolymers, etc., thereof, andblends and modifications thereof. The term “polyolefin” shall includeall possible structures thereof, which include, but are not limited to,isotatic, synodiotactic, and random symmetries. Copolymers includeatactic and block copolymers.

The term “superabsorbent materials” refers to water-swellable,water-insoluble organic or inorganic materials including superabsorbentpolymers and superabsorbent polymer compositions capable, under the mostfavorable conditions, of absorbing at least about 10 times their weight,or at least about 15 times their weight, or at least about 25 timestheir weight in an aqueous solution containing 0.9 weight percent sodiumchloride.

The term “superabsorbent polymer composition” refers to a superabsorbentpolymer comprising a surface additive in accordance with the presentinvention.

The terms “superabsorbent polymer” and “superabsorbent polymerpreproduct” refer to a material that is produced by conducting all ofthe steps for making a superabsorbent polymer as described herein, up toand including drying the material, and coarse grinding in a crusher.

The term “preproduct fines” refers to a material that is produced byconducting all of the steps for making a superabsorbent polymer asdescribed herein, up to and including drying the material, and coarsegrinding in a crusher, and removing particles greater than about 150microns.

The term “surface crosslinking” means that the level of functionalcrosslinks in the vicinity of the surface of the superabsorbent polymerparticle generally is higher than the level of functional crosslinks inthe interior of the superabsorbent polymer particle. As used herein,“surface” describes the outer-facing boundaries of the particle. Forporous superabsorbent polymer particles, exposed internal surface alsoare included in the definition of surface.

The term “thermoplastic” describes a material that softens when exposedto heat and which substantially returns to a non-softened condition whencooled to room temperature.

The term “% by weight” or “% wt” when used herein and referring tocomponents of the superabsorbent polymer composition, is to beinterpreted as based on the weight of the dry superabsorbent polymercomposition, unless otherwise specified herein.

These terms may be defined with additional language in the remainingportions of the specification.

DETAILED DESCRIPTION

An embodiment of the present invention includes a high-capacitysuperabsorbent polymer composition comprising superabsorbent polymerparticles surface treated with from about 0.01% to about 2% by weight ofan inorganic metal compound, based on the dry superabsorbent polymercomposition wherein the superabsorbent polymer composition exhibits aCentrifuge Retention Capacity of at least about 30 g/g and a free swellgel bed permeability of at least 10 Darcy as measured by the Free SwellGel Bed Permeability.

Another embodiment of the present invention includes a high-capacitysuperabsorbent polymer composition comprising a superabsorbent polymercomprising:

a) from about 55% to about 99.9% by weight of the superabsorbent polymerof polymerizable unsaturated acid group containing monomers based on thesuperabsorbent polymer; and

b) from about 0.001% to about 5% by weight of internal crosslinkingagent based on the polymerizable unsaturated acid group containingmonomer, wherein the superabsorbent polymer has a degree ofneutralization of greater than about 25%, wherein elements a) and b) arepolymerized and prepared into superabsorbent polymer particles furthercomprising the following surface additives to form surface-treatedsuperabsorbent polymer particles

-   -   i) from about 0.001% to about 5% by weight of surface        crosslinking agent based on the superabsorbent polymer        composition;    -   ii) from about 0.01% to about 2% by weight of a water-insoluble        inorganic metal compound based on the superabsorbent polymer        composition; and    -   iii) from 0% to about 5% by weight of a polymeric coating based        on the superabsorbent polymer composition

Another embodiment of the present invention comprises a process oftreating superabsorbent polymer particles with finely-divided,water-insoluble inorganic metal salt comprising the steps of a)supplying superabsorbent polymer particles; b) preparing a firstsolution of a first inorganic metal salt; c) adding to and mixing withthe first solution of b) a second solution of a second inorganic metalsalt, wherein the first solution and second solution react on mixing toprecipitate a third water-insoluble metal salt to form a water-insolublemetal salt slurry; d) optionally oxidizing the metal of thewater-insoluble metal salt slurry to a higher valence state; and e)applying the water-insoluble metal salt slurry to a superabsorbentpolymer particles without isolation and drying of the water-insolublemetal salt slurry.

Another embodiment of the present invention comprises a process oftreating superabsorbent polymer particles with finely-divided,water-insoluble inorganic metal salt comprising the steps of a)supplying superabsorbent polymer particles; b) preparing a firstsolution of a first inorganic metal salt; c) preparing a second solutionof a second inorganic metal salt; d) applying the first solution andsecond solution to the superabsorbent polymer particles to form awater-insoluble inorganic metal salt precipitate directly on or in thevicinity of the surface of the superabsorbent polymer particles.

A superabsorbent polymer as set forth in embodiments of the presentinvention is obtained by the initial polymerization of from about 55% toabout 99.9% by weight of the superabsorbent polymer of polymerizableunsaturated acid group containing monomer. A suitable monomer includesany of those containing carboxyl groups, such as acrylic acid,methacrylic acid, or 2-acrylamido-2-methylpropanesulfonic acid, ormixtures thereof. It is desirable for at least about 50% by weight, andmore desirable for at least about 75% by weight of the acid groups to becarboxyl groups.

The acid groups are neutralized to the extent of at least about 25 mol%, that is, the acid groups are desirably present as sodium, potassium,or ammonium salts. In some aspects, the degree of neutralization may beat least about 50 mol %. In some aspects, it is desirable to utilizepolymers obtained by polymerization of acrylic acid or methacrylic acid,the carboxyl groups of which are neutralized to the extent of from about50 mol % to about 80 mol %, in the presence of internal crosslinkingagents.

In some aspects, the suitable monomer that can be copolymerized with theethylenically unsaturated monomer may include, but is not limited toacrylamide, methacrylamide, hydroxyethyl acrylate,dimethylaminoalkyl(meth)-acrylate, ethoxylated(meth)-acrylates,dimethylaminopropylacrylamide, or acrylamidopropyltrimethylammoniumchloride. Such monomer may be present in a range of from 0% to about 40%by weight of the copolymerized monomer.

The superabsorbent polymer of the invention also includes internalcrosslinking agents. The internal crosslinking agent has at least twoethylenically unsaturated double bonds, or one ethylenically unsaturateddouble bond and one functional group that is reactive toward acid groupsof the polymerizable unsaturated acid group containing monomer, orseveral functional groups that are reactive towards acid groups can beused as the internal crosslinking component and is desirably presentduring the polymerization of the polymerizable unsaturated acid groupcontaining a monomer.

Examples of internal crosslinking agents include, but are not limitedto, aliphatic unsaturated amides, such as methylenebisacryl- or-methacrylamide or ethylenebisacrylamide; aliphatic esters of polyols oralkoxylated polyols with ethylenically unsaturated acids, such asdi(meth)acrylates or tri(meth)acrylates of butanediol or ethyleneglycol, polyglycols or trimethylolpropane; di- and triacrylate esters oftrimethylolpropane which may be oxyalkylated, desirably ethoxylated,with about 1 to about 30 moles of alkylene oxide; acrylate andmethacrylate esters of glycerol and pentaerythritol and of glycerol andpentaerythritol oxyethylated with desirably about 1 to about 30 mol ofethylene oxide; allyl compounds, such as allyl(meth)acrylate,alkoxylated allyl(meth)acrylate reacted with desirably about 1 to about30 mol of ethylene oxide, triallyl cyanurate, triallyl isocyanurate,maleic acid diallyl ester, poly-allyl esters, tetraallyloxyethane,triallylamine, tetraallylethylenediamine, diols, polyols, hydroxy allylor acrylate compounds and allyl esters of phosphoric acid or phosphorousacid; and monomers that are capable of crosslinking, such as N-methylolcompounds of unsaturated amides, such as of methacrylamide oracrylamide, and the ethers derived therefrom. Ionic crosslinkers such asmultivalent metal salts may also be employed. Mixtures of thecrosslinking agents mentioned can also be employed. The content of theinternal crosslinking agents is from about 0.001% to about 5% by weightsuch as from about 0.2% to about 3% by weight based on the total amountof the polymerizable unsaturated acid group containing monomer.

In some aspects, initiators can be used for initiation of thefree-radical polymerization. Suitable initiators include, but are notlimited to, azo or peroxo compounds, redox systems or UV initiators,sensitizers, and/or radiation.

After polymerization, the superabsorbent polymer is generally formedinto particles. The superabsorbent polymer particles may then be surfacecrosslinked after polymerization by the addition of a surfacecrosslinking agent and heat-treatment. In general, surface crosslinkingis a process that is believed to increase the crosslink density of thepolymer matrix in the vicinity of the superabsorbent particle surfacewith respect to the crosslinking density of the particle interior.

In some particular aspects, desirable surface crosslinking agentsinclude chemicals with one or more functional groups that are reactivetoward pendant groups of the polymer chains, typically the acid groups.The surface crosslinking agent may be present in an amount of from about0.001% to about 5% by weight of the dry superabsorbent polymercomposition, and such as from about 0.1% to about 3% by weight, and suchas from about 0.1% to about 1% by weight, based on the weight of the drysuperabsorbent polymer composition. Applicants have found that a heattreatment step after addition of the surface crosslinking agent isdesirable.

In one particular aspect, the particulate superabsorbent polymer iscoated or surface-treated with an alkylene carbonate followed by heatingto effect surface crosslinking, which can improve the surfacecrosslinking density and the gel strength characteristics of thesuperabsorbent polymer particle. More specifically, the surfacecrosslinking agent is coated onto the superabsorbent polymer particulateby mixing the polymer particulate with an aqueous alcoholic solution ofthe alkylene carbonate surface crosslinking agent. The amount of alcoholis determined by the solubility of the alkylene carbonate and is kept aslow as possible for various reasons. Suitable alcohols are methanol,isopropanol, ethanol, butanol, or butyl glycol, as well as mixtures ofthese alcohols. In some aspects, the solvent desirably is water, whichtypically is used in an amount of about 0.3% by weight to about 5.0% byweight, based on the weight of the dry superabsorbent polymer. In otheraspects, the alkylene carbonate surface crosslinking agent is dissolvedin water without any alcohol. In still other aspects, the alkylenecarbonate surface crosslinking agent may be applied from a powdermixture, for example, with an inorganic carrier material, such assilicone dioxide (SiO₂), or in a vapor state by sublimation of thealkylene carbonate.

To achieve the desired surface crosslinking properties, the alkylenecarbonate is distributed evenly on the particulate superabsorbentpolymer. For this purpose, mixing is effected in suitable mixers knownin the art, such as fluidized bed mixers, paddle mixers, rotary drummixers, or twin-worm mixers. It is also possible to carry out thecoating of the particulate superabsorbent polymer during one of theprocess steps in the production of the particulate superabsorbentpolymer. In one particular aspect, a suitable process for this purposeis the inverse suspension polymerization process.

The heat treatment, that may follow the coating treatment, may becarried out as follows. In general, the heat treatment is at atemperature of from about 100° C. to about 300° C. Lower temperaturesare possible if highly reactive epoxide crosslinking agents are used.However, if alkylene carbonates are used, then the thermal treatment issuitably at a temperature of from about 150° C. to about 250° C. In thisparticular aspect, the treatment temperature depends on the dwell timeand the kind of alkylene carbonate. For example, at a temperature ofabout 150° C., the thermal treatment is carried out for one hour orlonger. In contrast, at a temperature of about 250° C., a few minutes(e.g., from about 0.5 minutes to about 5 minutes) are sufficient toachieve the desired surface cross-linking properties. The thermaltreatment may be carried out in conventional dryers or ovens known inthe art.

While particles may be used by way of example of the physical form ofsuperabsorbent polymer composition, the invention is not limited to thisform and is applicable to other forms such as fibers, foams, films,beads, rods, and the like, as discussed above. In some aspects, when thesuperabsorbent polymer composition exists as particles or in granuleform, it is desirable that these particles have a size of from about 150μm to about 850 μm based on the sieving process that is well known inthe superabsorbent industry.

In some aspects, the superabsorbent polymer composition of the presentinvention includes from 0% to about 5% by weight, and from about 0.001%to about 5% by weight, and from about 0.01% to about 0.5% by weight ofthe dry superabsorbent polymer composition of a polymeric coating, suchas a thermoplastic coating, or a cationic coating, or a combination of athermoplastic coating and a cationic coating. In some particularaspects, the polymeric coating desirably is a polymer that may be in asolid, emulsion, suspension, colloidal, or solubilized state, orcombinations thereof. Polymeric coatings suitable for this invention mayinclude, but are not limited to, a thermoplastic coating having athermoplastic melt temperature wherein the polymeric coating is appliedto the particle surface coincident with or followed by a temperature ofthe treated superabsorbent polymer particle at about the thermoplasticmelt temperature.

Examples of thermoplastic polymers include, but are not limited to,polyolefin, polyethylene, polyester, polyamide, polyurethane, styrenepolybutadiene, linear low density polyethylene (LLDPE), ethylene acrylicacid copolymer (EAA), ethylene alkyl methacrylate copolymer (EMA),polypropylene (PP), maleated polypropylene, ethylene vinyl acetatecopolymer (EVA), polyester, polyamide, and blends of all families ofpolyolefins, such as blends of PP, EVA, EMA, EEA, EBA, HDPE, MDPE, LDPE,LLDPE, and/or VLDPE, may also be advantageously employed. The termpolyolefin as used herein is defined above. In particular aspects, theApplicants have found that maleated polypropylene to be a desirablethermoplastic polymer for use in the present invention. A thermoplasticpolymer may be functionalized to have additional benefits such as watersolubility or dispersability.

Polymeric coatings of this invention may also include a cationicpolymer. A cationic polymer as used herein refers to a polymer ormixture of polymers comprising a functional group or groups having apotential of becoming positively charged ions upon ionization in anaqueous solution. Suitable functional groups for a cationic polymerinclude, but are not limited to, primary, secondary, or tertiary aminogroups, imino groups, imido groups, amido groups, and quaternaryammonium groups. Examples of synthetic cationic polymers include, butare not limited to, the salts or partial salts of poly(vinyl amines),poly(allylamines), poly(ethylene imine), poly(amino propanol vinylethers), poly(acrylamidopropyl trimethyl ammonium chloride),poly(diallyldimethyl ammonium chloride). Poly(vinyl amines) include, butare not limited to, LUPAMIN® 9095 available from BASF Corporation, MountOlive, N.J. Examples of natural-based cationic polymers include, but arenot limited to, partially deacetylated chitin, chitosan, and chitosansalts. Synthetic polypeptides such as polyasparagins, polylysines,polyglutamines, and polyarginines are also suitable cationic polymers.

The superabsorbent polymer compositions according to the invention mayinclude from about 0.01% to about 2% by weight, or from about 0.01% toabout 1% by weight based on the dry superabsorbent polymer compositionof a water-insoluble inorganic metal compound. The water-insolubleinorganic metal compound may include, but are not limited to, a cationselected from aluminum, titanium, calcium, or iron and an anion selectedfrom phosphate, borate, or chromate. Examples of water-insolubleinorganic metal compounds include aluminum phosphate and an insolublemetal borate. The insoluble metal borate is selected from titaniumborate, aluminum borate, iron borate, magnesium borate, manganeseborate, or calcium borate. The chemical formula TiBO will be used hereinto designate titanium borate and analogous compounds such as titanium(III) borate TiBO₃. In addition, the chemical formulation alsodesignates the case when titanium (III) borate TiBO₃ is treated withhydrogen peroxide to obtain titanium (IV) borate. The inorganic metalcompound may have a mass median particle size of less than about 2 μm,and may have a mass median particle size of less than about 1 μm.

The inorganic metal compound can be applied in the dry physical form tothe surface of the superabsorbent polymer particles. For this, thesuperabsorbent polymer particles can be intimately mixed with the finelydivided inorganic metal compound. The finely divided inorganic metalcompound is usually added at about room temperature to thesuperabsorbent polymer particles and mixed in until a homogeneousmixture is present. For this purpose, mixing is effected in suitablemixers known in the art, such as fluidized bed mixers, paddle mixers,rotary drum mixers, or twin-worm mixers. The mixing of thesuperabsorbent polymer particles with the finely divided water-insolubleinorganic metal compound may take place before or after any surfacecrosslinking, for example during the application of the surfacecrosslinking agent.

Alternatively, a suspension of a finely divided water-insolubleinorganic metal compounds can be prepared and applied to a particulatewater absorbent polymer. The suspension is applied, for example, byspraying. Useful dispersion media for preparing the suspension includewater, organic solvents such as alcohols, for example methanol, ethanol,isopropanol, ketones, for example acetone, methyl ethyl ketone, ormixtures of water with the aforementioned organic solvents. Other usefuldispersion media include dispersion aids, surfactants, protectivecolloidals, viscosity modifiers, and other auxiliaries to assist in thepreparation of the suspension. The suspension can be applied inconventional reaction mixers or mixing and drying systems as describedabove at a temperature in the range from room temperature to less thanthe boiling point of the dispersion medium, preferably at about roomtemperature. It is appropriate to combine the application of thesuspension with a surface crosslinking step by dispersing the finelydivided water-insoluble metal salt in the solution of the surfacecrosslinking agent. Alternatively, the suspension can also be appliedbefore or after the surface crosslinking step. The application of theslurry may be followed by a drying step.

In some aspects, the superabsorbent polymer compositions according tothe invention can include from 0% to about 5%, or in the alternativefrom about 0.01% to about 3%, by weight of the dry superabsorbentpolymer composition of silica. Examples of silica include fumed silica,precipitated silica, silicon dioxide, silicic acid, and silicates. Insome particular aspects, microscopic noncrystalline silicon dioxide isdesirable. Products include SIPERNAT 22S and AEROSIL 200 available fromDegussa Corporation, Parsippany, N.J. In some aspects, the particlediameter of the inorganic powder can be 1,000 μm or smaller, such as 100μm or smaller.

In some aspects, the superabsorbent polymer compositions may alsoinclude from 0% to about 30% by weight of the dry superabsorbent polymercomposition, such as from about 0.1% to about 5% by weight, ofwater-soluble polymers based by weight of the dry superabsorbent polymercomposition, of partly or completely hydrolyzed polyvinyl acetate,polyvinylpyrrolidone, starch or starch derivatives, polyglycols,polyethylene oxides, polypropylene oxides, or polyacrylic acids.

In some aspects, additional surface additives may optionally be employedwith the superabsorbent polymer particles, such as odor-bindingsubstances, such as cyclodextrins, zeolites, inorganic or organic salts,and similar materials; anti-caking additives, flow modification agents,surfactants, viscosity modifiers, and the like. In addition, surfaceadditives may be employed that perform several roles during surfacemodifications. For example, a single additive may be a surfactant,viscosity modifier, and may react to crosslink polymer chains.

In some aspects, the superabsorbent polymer compositions of the presentinvention may be, after a heat treatment step, treated with water sothat the superabsorbent polymer composition has a water content of up toabout 10% by weight of the superabsorbent polymer composition. Thiswater may be added with one or more of the surface additives from aboveadded to the superabsorbent polymer.

The superabsorbent polymer compositions according to the invention aredesirably prepared by two methods. The composition can be preparedcontinuously or discontinuously in a large-scale industrial manner, theafter-crosslinking according to the invention being carried outaccordingly.

According to one method, the partially neutralized monomer, such asacrylic acid, is converted into a gel by free-radical polymerization inaqueous solution in the presence of crosslinking agents and any furthercomponents, and the gel is comminuted, dried, ground, and sieved off tothe desired particle size. This polymerization can be carried outcontinuously or discontinuously. For the present invention, the size ofthe high-capacity superabsorbent polymer composition particles isdependent on manufacturing processes including milling and sieving. Itis well known to those skilled in the art that particle sizedistribution of the superabsorbent polymer particles resembles a normaldistribution or a bell shaped curve. It is also known that for variousreasons, the normal distribution of the particle size distribution maybe skewed in either direction.

The superabsorbent polymer particles of the present invention generallyinclude particle sizes ranging from about 50 to about 1000 microns, orfrom about 150 to about 850 microns. The present invention may includeat least about 40 wt % of the particles having a particle size fromabout 300 μm to about 600 μm, at least about 50 wt % of the particleshaving a particle size from about 300 μm to about 600 μm, or at leastabout 60 wt % of the particles having a particle size from about 300 μmto about 600 μm as measured by screening through a U.S. standard 30 meshscreen and retained on a U.S. standard 50 mesh screen. In addition, thesize distribution of the superabsorbent polymer particles of the presentinvention may include less than about 30% by weight of particles havinga size greater than about 600 microns, and less than about 30% by weightof particles having a size of less than about 300 microns as measuredusing for example a RO-TAP® Mechanical Sieve Shaker Model B availablefrom W. S. Tyler, Inc., Mentor Ohio.

According to another method, inverse suspension and emulsionpolymerization can also be used for preparation of the productsaccording to the invention. According to these processes, an aqueous,partly neutralized solution of monomer, such as acrylic acid, isdispersed in a hydrophobic, organic solvent with the aid of protectivecolloids and/or emulsifiers, and the polymerization is started by freeradical initiators. The internal crosslinking agents may be eitherdissolved in the monomer solution and are metered in together with this,or are added separately and optionally during the polymerization. Theaddition of a water-soluble polymer as the graft base optionally takesplace via the monomer solution or by direct introduction into theorganic solvent. The water is then removed azeotropically from themixture, and the polymer is filtered off and optionally dried. Internalcrosslinking can be carried out by polymerizing-in a polyfunctionalcrosslinking agent dissolved in the monomer solution and/or by reactionof suitable crosslinking agents with functional groups of the polymerduring the polymerization steps.

The result of these methods is a superabsorbent preproduct. Asuperabsorbent preproduct as used herein is produced by repeating all ofthe steps for making the superabsorbent, up to and including drying thematerial, and coarse grinding in a crusher, and removing particlesgreater than about 850 microns and smaller than about 150 microns.

The superabsorbent polymer composition of the present invention exhibitscertain characteristics, or properties, as measured by Free Swell GelBed Permeability (GBP), Centrifuge Retention Capacity (CRC), andabsorbency under load at about 0.9 psi (AUL(0.9 psi)). The Free SwellGel Bed Permeability (GBP) Test is a measurement of the permeability ofa swollen bed of superabsorbent material in Darcy (e.g., separate fromthe absorbent structure) under a confining pressure after what iscommonly referred to as “free swell” conditions. In this context, theterm “free swell” means that the superabsorbent material is allowed toswell without a swell restraining load upon absorbing test solution aswill be described.

The Centrifuge Retention Capacity (CRC) Test measures the ability of thesuperabsorbent composition to retain liquid therein after beingsaturated and subjected to centrifugation under controlled conditions.The resultant retention capacity is stated as grams of liquid retainedper gram weight of the sample (g/g).

The superabsorbent polymer compositions according to the presentinvention can be employed in many products including sanitary towels,diapers, or wound coverings, and they have the property that theyrapidly absorb large amounts of menstrual blood, urine, or other bodyfluids. Since the agents according to the invention retain the absorbedliquids even under pressure and are also capable of distributing furtherliquid within the construction in the swollen state, they are moredesirably employed in higher concentrations, with respect to thehydrophilic fiber material, such as fluff, when compared to conventionalcurrent superabsorbent compositions. They are also suitable for use as ahomogeneous superabsorber layer without fluff content within the diaperconstruction, as a result of which particularly thin articles arepossible. The polymers are furthermore suitable for use in hygienearticles (incontinence products) for adults.

The preparation of laminates in the broadest sense, and of extruded andcoextruded, wet- and dry-bonded, as well as subsequently bondedstructures, are possible as further preparation processes. A combinationof these possible processes with one another is also possible.

The superabsorbent polymer compositions according to the invention mayalso be employed in absorbent articles that are suitable for furtheruses. In particular, the superabsorbent polymer compositions of thisinvention can be used in absorbent compositions for absorbents for wateror aqueous liquids, desirably in constructions for absorption of bodyfluids, in foamed and non-foamed sheet-like structures, in packagingmaterials, in constructions for plant growing, as soil improvementagents, or as active compound carriers. For this, they are processedinto a web by mixing with paper or fluff or synthetic fibers or bydistributing the superabsorbent polymer composition particles betweensubstrates of paper, fluff, or non-woven textiles, or by processing intocarrier materials. They are further suited for use in absorbentcompositions such as wound dressings, packaging, agriculturalabsorbents, food trays and pads, and the like.

The superabsorbent polymer compositions according to the invention showa significant improvement in permeability, i.e. an improvement in thetransportation of liquid in the swollen state, while maintaining highabsorption and retention capacity, as compared to known superabsorbentpolymer compositions.

The present invention may be better understood with reference to thefollowing examples.

Test Procedures

Free-Swell Gel Bed Permeability Test (FSGBP)

As used herein, the Free-Swell Gel Bed Permeability Test, also referredto as the Gel Bed Permeability (GBP) Under 0 psi Swell Pressure Test,determines the permeability of a swollen bed of gel particles (e.g.,such as the surface treated absorbent material or the superabsorbentmaterial prior to being surface treated), under what is commonlyreferred to as “free swell” conditions. The term “free swell” means thatthe gel particles are allowed to swell without a restraining load uponabsorbing test solution as will be described. A suitable apparatus forconducting the Gel Bed Permeability Test is shown in FIGS. 1, 2 and 3and indicated generally as 500. The test apparatus assembly 528comprises a sample container, generally indicated at 530, and a plunger,generally indicated at 536. The plunger comprises a shaft 538 having acylinder hole bored down the longitudinal axis and a head 550 positionedat the bottom of the shaft. The shaft hole 562 has a diameter of about16 mm. The plunger head is attached to the shaft, such as by adhesion.Twelve holes 544 are bored into the radial axis of the shaft, threepositioned at every 90 degrees having diameters of about 6.4 mm. Theshaft 538 is machined from a LEXAN rod or equivalent material and has anouter diameter of about 2.2 cm and an inner diameter of about 16 mm.

The plunger head 550 has a concentric inner ring of seven holes 560 andan outer ring of 14 holes 554, all holes having a diameter of about 8.8millimeters as well as a hole of about 16 mm aligned with the shaft. Theplunger head 550 is machined from a LEXAN rod or equivalent material andhas a height of approximately 16 mm and a diameter sized such that itfits within the cylinder 534 with minimum wall clearance but stillslides freely. The total length of the plunger head 550 and shaft 538 isabout 8.25 cm, but can be machined at the top of the shaft to obtain thedesired mass of the plunger 536. The plunger 536 comprises a 100 meshstainless steel cloth screen 564 that is biaxially stretched to tautnessand attached to the lower end of the plunger 536. The screen is attachedto the plunger head 550 using an appropriate solvent that causes thescreen to be securely adhered to the plunger head 550. Care must betaken to avoid excess solvent migrating into the open portions of thescreen and reducing the open area for liquid flow. Acrylic solventWeld-on 4 from IPS Corporation (having a place of business in Gardena,Calif., USA) is a suitable solvent.

The sample container 530 comprises a cylinder 534 and a 400 meshstainless steel cloth screen 566 that is biaxially stretched to tautnessand attached to the lower end of the cylinder 534. The screen isattached to the cylinder using an appropriate solvent that causes thescreen to be securely adhered to the cylinder. Care must be taken toavoid excess solvent migrating into the open portions of the screen andreducing the open area for liquid flow. Acrylic solvent Weld-on 4 fromIPS Corporation (having a place of business in Gardena, Calif., USA) isa suitable solvent. A gel particle sample, indicated as 568 in FIG. 2,is supported on the screen 566 within the cylinder 534 during testing.

The cylinder 534 may be bored from a transparent LEXAN rod or equivalentmaterial, or it may be cut from a LEXAN tubing or equivalent material,and has an inner diameter of about 6 cm (e.g., a cross-sectional area ofabout 28.27 cm²), a wall thickness of about 0.5 cm and a height ofapproximately 7.95 cm. A step is machined into the outer diameter of thecylinder 534 such that a region 534 a with an outer diameter of 66 mmexists for the bottom 31 mm of the cylinder 534. An O-ring 540 whichfits the diameter of region 534 a may be placed at the top of the step.

The annular weight 548 has a counter-bored hole about 2.2 cm in diameterand 1.3 cm deep so that it slips freely onto the shaft 538. The annularweight also has a thru-bore 548 a of about 16 mm. The annular weight 548can be made from stainless steel or from other suitable materialsresistant to corrosion in the presence of the test solution, which is0.9 weight percent sodium chloride solution in distilled water. Thecombined weight of the plunger 536 and annular weight 548 equalsapproximately 596 grams (g), which corresponds to a pressure applied tothe sample 568 of about 0.3 pounds per square inch (psi), or about 20.7dynes/cm² (2.07 kPa), over a sample area of about 28.27 cm².

When the test solution flows through the test apparatus during testingas described below, the sample container 530 generally rests on a weir600. The purpose of the weir is to divert liquid that overflows the topof the sample container 530 and diverts the overflow liquid to aseparate collection device 601. The weir can be positioned above a scale602 with a beaker 603 resting on it to collect saline solution passingthrough the swollen sample 568.

To conduct the Gel Bed Permeability Test under “free swell” conditions,the plunger 536, with the weight 548 seated thereon, is placed in anempty sample container 530 and the height from the top of the weight 548to the bottom of the sample container 530 is measured using a suitablegauge accurate to 0.01 mm. The force the thickness gauge applies duringmeasurement should be as low as possible, preferably less than about0.74 Newtons. It is important to measure the height of each empty samplecontainer 530, plunger 536, and weight 548 combination and to keep trackof which plunger 536 and weight 548 is used when using multiple testapparatus. The same plunger 536 and weight 548 should be used formeasurement when the sample 568 is later swollen following saturation.It is also desirable that the base that the sample cup 530 is resting onis level, and the top surface of the weight 548 is parallel to thebottom surface of the sample cup 530.

The sample to be tested is prepared from superabsorbent polymercomposition particles which are prescreened through a U.S. standard 30mesh screen and retained on a U.S. standard 50 mesh screen. As a result,the test sample comprises particles sized in the range of about 300 toabout 600 microns. The superabsorbent polymer particles can bepre-screened with, for example, a RO-TAP Mechanical Sieve Shaker Model Bavailable from W. S. Tyler, Inc., Mentor Ohio. Sieving is conducted for10 minutes. Approximately 2.0 grams of the sample is placed in thesample container 530 and spread out evenly on the bottom of the samplecontainer. The container, with 2.0 grams of sample in it, without theplunger 536 and weight 548 therein, is then submerged in the 0.9% salinesolution for a time period of about 60 minutes to saturate the sampleand allow the sample to swell free of any restraining load. Duringsaturation, the sample cup 530 is set on a mesh located in the liquidreservoir so that the sample cup 530 is raised slightly above the bottomof the liquid reservoir. The mesh does not inhibit the flow of salinesolution into the sample cup 530. A suitable mesh can be obtained aspart number 7308 from Eagle Supply and Plastic, having a place ofbusiness in Appleton, Wis., U.S.A. Saline does not fully cover thesuperabsorbent polymer composition particles, as would be evidenced by aperfectly flat saline surface in the test cell. Also, saline depth isnot allowed to fall so low that the surface within the cell is definedsolely by swollen superabsorbent, rather than saline.

At the end of this period, the plunger 536 and weight 548 assembly isplaced on the saturated sample 568 in the sample container 530 and thenthe sample container 530, plunger 536, weight 548, and sample 568 areremoved from the solution. After removal and before being measured, thesample container 530, plunger 536, weight 548, and sample 568 are toremain at rest for about 30 seconds on a suitable flat, large gridnon-deformable plate of uniform thickness. The thickness of thesaturated sample 568 is determined by again measuring the height fromthe top of the weight 548 to the bottom of the sample container 530,using the same thickness gauge used previously provided that the zeropoint is unchanged from the initial height measurement. The samplecontainer 530, plunger 536, weight 548, and sample 568 may be placed ona flat, large grid non-deformable plate of uniform thickness that willprevent liquid in the sample container from being released onto a flatsurface due to surface tension. The plate has an overall dimension of7.6 cm by 7.6 cm, and each grid has a cell size dimension of 1.59 cmlong by 1.59 cm wide by 1.12 cm deep. A suitable flat, large gridnon-deformable plate material is a parabolic diffuser panel, cataloguenumber 1624K27, available from McMaster Can Supply Company, having aplace of business in Chicago, Ill., U.S.A., which can then be cut to theproper dimensions. This flat, large mesh non-deformable plate must alsobe present when measuring the height of the initial empty assembly. Theheight measurement should be made as soon as practicable after thethickness gauge is engaged. The height measurement obtained frommeasuring the empty sample container 530, plunger 536, and weight 548 issubtracted from the height measurement obtained after saturating thesample 568. The resulting value is the thickness, or height “H” of theswollen sample.

The permeability measurement is initiated by delivering a flow of the0.9% saline solution into the sample container 530 with the saturatedsample 568, plunger 536, and weight 548 inside. The flow rate of testsolution into the container is adjusted to cause saline solution tooverflow the top of the cylinder 534 thereby resulting in a consistenthead pressure equal to the height of the sample container 530. The testsolution may be added by any suitable means that is sufficient to ensurea small, but consistent amount of overflow from the top of the cylinder,such as with a metering pump 604. The overflow liquid is diverted into aseparate collection device 601. The quantity of solution passing throughthe sample 568 versus time is measured gravimetrically using the scale602 and beaker 603. Data points from the scale 602 are collected everysecond for at least sixty seconds once the overflow has begun. Datacollection may be taken manually or with data collection software. Theflow rate, Q, through the swollen sample 568 is determined in units ofgrams/second (g/s) by a linear least-square fit of fluid passing throughthe sample 568 (in grams) versus time (in seconds).

Permeability in cm² is obtained by the following equation:K=[Q*H*μ]/[A*ρ*P], where K=Permeability (cm²), Q=flow rate (g/sec),H=height of swollen sample (cm), μ=liquid viscosity (poise)(approximately one centipoise for the test solution used with thisTest), A=cross-sectional area for liquid flow (28.27 cm² for the samplecontainer used with this Test), ρ=liquid density (g/cm³) (approximatelyone g/cm³, for the test solution used with this Test) and P=hydrostaticpressure (dynes/cm²) (normally approximately 7,797 dynes/cm²). Thehydrostatic pressure is calculated from P=ρ*g*h, where ρ=liquid density(g/cm³), g=gravitational acceleration, nominally 981 cm/sec², andh=fluid height, e.g., 7.95 cm for the Gel Bed Permeability Testdescribed herein.

A minimum of two samples is tested and the results are averaged todetermine the gel bed permeability of the sample.

Water Content

The amount of water content, measured as “% moisture,” can be measuredas follows: 1) Weigh 4.5-5.5 grams of superabsorbent polymer composition(SAP) accurately in a pre-weighed aluminum weighing pan; 2) place theSAP and pan into a standard lab oven preheated to 150° C. for 30minutes; 3) remove and re-weigh the pan and contents; and 4) calculatethe percent moisture using the following formula:% Moisture={((pan wt+initial SAP wt)−(dried SAP & pan wt))*100}/driedSAP wt

Centrifuge Retention Capacity Test

The Centrifuge Retention Capacity (CRC) Test measures the ability of thesuperabsorbent polymer to retain liquid therein after being saturatedand subjected to centrifugation under controlled conditions. Theresultant retention capacity is stated as grams of liquid retained pergram weight of the sample (g/g). The sample to be tested is preparedfrom particles that are pre-screened through a U.S. standard 30-meshscreen and retained on a U.S. standard 50-mesh screen. As a result, thesuperabsorbent polymer sample comprises particles sized in the range ofabout 300 to about 600 microns. The particles can be pre-screened byhand or automatically.

The retention capacity is measured by placing about 0.2 grams of thepre-screened superabsorbent polymer sample into a water-permeable bagthat will contain the sample while allowing a test solution (0.9 weightpercent sodium chloride in distilled water) to be freely absorbed by thesample. A heat-sealable tea bag material, such as that available fromDexter Corporation (having a place of business in Windsor Locks, Conn.,U.S.A.) as model designation 1234T heat sealable filter paper works wellfor most applications. The bag is formed by folding a 5-inch by 3-inchsample of the bag material in half and heat-sealing two of the openedges to form a 2.5-inch by 3-inch rectangular pouch. The heat seals areabout 0.25 inches inside the edge of the material. After the sample isplaced in the pouch, the remaining open edge of the pouch is alsoheat-sealed. Empty bags are also made to serve as controls. Threesamples are prepared for each superabsorbent polymer composition to betested.

The sealed bags are submerged in a pan containing the test solution atabout 23° C., making sure that the bags are held down until they arecompletely wetted. After wetting, the samples remain in the solution forabout 30 minutes, at which time they are removed from the solution andtemporarily laid on a non-absorbent flat surface.

The wet bags are then placed into the basket wherein the wet bags areseparated from each other and are placed at the outer circumferentialedge of the basket, wherein the basket is of a suitable centrifugecapable of subjecting the samples to a g-force of about 350. Onesuitable centrifuge is a CLAY ADAMS DYNAC II, model #0103, having awater collection basket, a digital rpm gauge, and a machined drainagebasket adapted to hold and drain the flat bag samples. Where multiplesamples are centrifuged, the samples are placed in opposing positionswithin the centrifuge to balance the basket when spinning. The bags(including the wet, empty bags) are centrifuged at about 1,600 rpm(e.g., to achieve a target g-force of about 290 g force with a variancefrom about 280 to about 300 g force), for 3 minutes. G force is definedas an unit, of inertial force on a body that is subjected to rapidacceleration or gravity, equal to 32 ft/sec² at sea level. The bags areremoved and weighed, with the empty bags (controls) being weighed first,followed by the bags containing the superabsorbent polymer compositionsamples. The amount of solution retained by the superabsorbent polymersample, taking into account the solution retained by the bag itself, isthe centrifuge retention capacity (CRC) of the superabsorbent polymer,expressed as grams of fluid per gram of superabsorbent polymer. Moreparticularly, the retention capacity is determined by the followingequation:

$\frac{\begin{matrix}{{{{sample}/{bag}}\mspace{14mu}{after}\mspace{14mu}{centrifuge}} -} \\{{{empty}\mspace{14mu}{bag}\mspace{14mu}{after}\mspace{14mu}{centrifuge}} - {{dry}\mspace{14mu}{sample}\mspace{14mu}{weight}}}\end{matrix}}{{dry}\mspace{14mu}{sample}\mspace{14mu}{weight}}$

The three samples are tested, and the results are averaged to determinethe Centrifuge Retention Capacity (CRC) of the superabsorbent polymercomposition.

Absorbency Under Load Test (AUL0.9 psi)

The Absorbency Under Load (AUL) Test measures the ability of thesuperabsorbent polymer composition particles to absorb a 0.9 weightpercent solution of sodium chloride in distilled water at roomtemperature (test solution) while the material is under a 0.9 psi load.The apparatus for testing AUL consists of:

-   -   An AUL assembly including a cylinder, a 4.4 g piston, and a        standard 317 gm weight. The components of this assembly are        described in additional detail below.    -   A flat-bottomed square plastic tray that is sufficiently broad        to allow the glass frits to lay on the bottom without contact        with the tray walls. A plastic tray that is 9″ by 9″(22.9        cm×22.9 cm), with a depth of 0.5 to 1″(1.3 cm to 2.5 cm) is        commonly used for this test method.    -   A 12.5 cm diameter sintered glass frit with a ‘C’ porosity        (25-50 microns). This frit is prepared in advance through        equilibration in saline (0.9% sodium chloride in distilled        water, by weight). In addition to being washed with at least two        portions of fresh saline, the frit must be immersed in saline        for at least 12 hours prior to AUL measurements.    -   Whatman Grade 1, 12.5 cm diameter filter paper circles.    -   A supply of saline (0.9% sodium chloride in distilled water, by        weight).

Referring to FIG. 4, the cylinder 412 of the AUL assembly 400 used tocontain the superabsorbent polymer composition particles 410 is madefrom one-inch (2.54 cm) inside diameter thermoplastic tubingmachined-out slightly to be sure of concentricity. After machining, a400 mesh stainless steel wire cloth 414 is attached to the bottom of thecylinder 412 by heating the steel wire cloth 414 in a flame until redhot, after which the cylinder 412 is held onto the steel wire clothuntil cooled. A soldering iron can be utilized to touch up the seal ifunsuccessful or if it breaks. Care must be taken to maintain a flatsmooth bottom and not distort the inside of the cylinder 412.

The 4.4 g piston (416) is made from one-inch diameter solid material(e.g., PLEXIGLAS®) and is machined to closely fit without binding in thecylinder 412.

A standard 317 gm weight 418 is used to provide a 62,053 dyne/cm² (about0.9 psi) restraining load. The weight is a cylindrical, 1 inch(2.5 cm)diameter, stainless steel weight that is machined to closely fit withoutbinding in the cylinder.

Unless specified otherwise, a sample 410 corresponding to a layer of atleast about 300 gsm. (0.16 g) of superabsorbent polymer compositionparticles is utilized for testing the AUL. The sample 410 is taken fromsuperabsorbent polymer composition particles that are pre-screenedthrough U.S. standard #30 mesh and retained on U.S. std. #50 mesh. Thesuperabsorbent polymer composition particles can be pre-screened with,for example, a RO-TAP® Mechanical Sieve Shaker Model B available from W.S. Tyler, Inc., Mentor Ohio. Sieving is conducted for about 10 minutes.

The inside of the cylinder 412 is wiped with an antistatic cloth priorto placing the superabsorbent polymer composition particles 410 into thecylinder 412.

The desired amount of the sample of sieved superabsorbent polymercomposition particles 410 (about 0.16 g) is weighed out on a weigh paperand evenly distributed on the wire cloth 414 at the bottom of thecylinder 412. The weight of the superabsorbent polymer compositionparticles in the bottom of the cylinder is recorded as ‘SA,’ for use inthe AUL calculation described below. Care is taken to be sure nosuperabsorbent polymer particles cling to the wall of the cylinder.After carefully placing the 4.4 g piston 412 and 317 g weight 418 on thesuperabsorbent polymer composition particles 410 in the cylinder 412,the AUL assembly 400 including the cylinder, piston, weight, andsuperabsorbent polymer composition particles is weighed, and the weightis recorded as weight ‘A’.

A sintered glass frit 424 (described above) is placed in the plastictray 420, with saline 422 added to a level equal to that of the uppersurface of the glass frit 424. A single circle of filter paper 426 isplaced gently on the glass frit 424, and the AUL assembly 400 with thesuperabsorbent polymer composition particles 410 is then placed on topof the filter paper 426. The AUL assembly 400 is then allowed to remainon top of the filter paper 426 for a test period of one hour, withattention paid to keeping the saline level in the tray constant. At theend of the one hour test period, the AUL apparatus is then weighed, withthis value recorded as weight ‘B.’

The AUL(0.9 psi) is calculated as follows:AUL(0.9 psi)=(B−A)/SA

wherein

A=Weight of AUL Unit with dry SAP

B=Weight of AUL Unit with SAP after 60 minutes absorption

SA=Actual SAP weight

A minimum of two tests is performed and the results are averaged todetermine the AUL value under 0.9 psi load. The samples are tested atabout 23° C. and about 50% relative humidity.

EXAMPLES

The following examples and preproducts for the examples are provided toillustrate the invention and do not limit the scope of the claims.Unless otherwise stated all parts, and percentages are by weight.

Preproduct Fines

Into a polyethylene vessel equipped with an agitator and cooling coilswas added, 25.0 kg of 50% NaOH to 37 kg of distilled water and cooled to20° C. 9.6 kg of glacial acrylic acid was then added to the causticsolution and the solution again cooled to 20′C. 47.8 g of polyethyleneglycol monoallylether acrylate, 47.8 g of ethoxylated trimethylolpropane triacrylate SARTOMER® 454 product, and 19.2 kg of glacialacrylic acid were added to the first solution, followed by cooling to4-6° C. Nitrogen was bubbled through the monomer solution for about 10minutes. The monomer solution was then discharged in 7.7 kg batches intorectangular trays. To each batch 80 g of 1% by weight of H₂O₂ aqueoussolution, 120 g of 2 wt % aqueous sodium persulfate solution, and 72 gof 0.5 wt % aqueous sodium erythorbate solution was added homogeneouslyinto the monomer solution stream by injection of the sodium erythorbatesolution into the stream of the monomer solution being conveyed from themonomer tank into a tray. The initiated monomer was allowed topolymerize for 20 minutes prior to grinding and drying at 175° C. Theproduct was sieved with an Minox MTS 600DS3V to remove particles ofremove particles greater than 850 microns and smaller than 150 microns.The particles that are smaller than 150 microns are Preproduct Fines.

Preproduct A

Into a polyethylene vessel equipped with an agitator and cooling coilswas added, 25.0 kg of 50% NaOH to 37 kg of distilled water and cooled to20° C. 9.6 kg of glacial acrylic acid was then added to the causticsolution and the solution again cooled to 20′C. 47.8 g of polyethyleneglycol monoallylether acrylate, 47.8 g of ethoxylated trimethylolpropane triacrylate SARTOMER® 454 product, and 19.2 kg of glacialacrylic acid were added to the first solution, followed by cooling to4-6° C. Nitrogen was bubbled through the monomer solution for about 10minutes, followed by the addition of 1.88 kg of Preproduct Fines. Themonomer solution was then discharged in 7.7 kg batches into rectangulartrays. To each batch 80 g of 1% by weight of H₂O₂ aqueous solution, 120g of 2 wt % aqueous sodium persulfate solution, and 72 g of 0.5 wt %aqueous sodium erythorbate solution was added homogeneously into themonomer solution stream by injection of the sodium erythorbate solutioninto the stream of the monomer solution being conveyed from the monomertank into a tray. The initiated monomer was allowed to polymerize for 20minutes. The resulting gel was chopped and extruded with a Hobart 4M6commercial extruder, followed by drying in a Procter & Schwartz Model062 forced air oven at 175° C. for 10 minutes with up flow and 6 minuteswith down flow air on a 20 in×40 in perforated metal tray to a finalproduct moisture level of less than 5 wt %. The dried material wascoarse-ground in a Prodeva Model 315-S crusher, milled in an MPI 666-Fthree-stage roller mill and sieved with an Minox MTS 600DS3V to removeparticles greater than 850 μm and smaller than 150 μm.

Preproduct AlPO₄ Coating Slurry

1269 g of aluminum sulfate tetradecahydrate were dissolved in 1500 gdeionized water having a temperature of about 85° C. to about 95° C. 860g of trisodium phosphate was dissolved in 1200 g of hot deionized water.The aluminum sulfate solution is then rapidly poured into the trisodiumphosphate solution. The resulting slurry was rapidly blended with 10 gPLURONIC® 25R2 surfactant which is available from BASF Corporation,Mount Olive, N.J., and sufficient 50% NaOH was added to bring the pH toneutral (pH of 7). 1413 g of pure ethylene carbonate was added to thesolution to bring the net weight to 6.14 kg. The liquid slurry wasfiltered through a 100 mesh screen to remove any large particles priorto spraying onto Preproduct A.

AlPO₄ Presscake

1269 g of aluminum sulfate tetradecahydrate were dissolved in 1500 g hotdeionized water. 860 g of trisodium phosphate was dissolved in 1200 g ofhot deionized water having a temperature of from about 85° C. to about95° C. The aluminum sulfate solution is then rapidly poured into thetrisodium phosphate solution. The resulting slurry was rapidly blendedwith 10 g PLURONIC® 25R2 surfactant, and sufficient 50% NaOH was addedto bring the pH to neutral (pH of 7). After precipitation of the AlPO₄,the suspension was filtered in a Büchner funnel and the resulting cakewas washed with 3 500 mL additions of DI water. The presscake wasmeasured as 20 wt % solids by oven drying.

Titanium Borate Presscake

200 g of disodium tetraborate was dissolved in 800 ml of hot deionizedwater. The water is in the temperature range of about 85° C. to about95° C. 150 g of 45% titanium III sulfate was added to the above solutionto form a dark slurry. 35% H₂O₂ was added drop-wise to the slurry untila homogenous yellow suspension resulted. The product was filtered in aBüchner funnel, and the cake was washed with deionized water. Solidswere determined to be 19.8% by oven drying.

Comparative Examples 1 & 2 and Examples 1-7

Preproduct A was coated with 1 wt % ethylene carbonate and 4 wt % waterusing a 20 wt % aqueous solution. The coated Preproduct A was fed at arate of 60-70 grams/minute into a continuous paddle reactor with a peaktemperature of 215° C. and a residence time of about 50 minutes toaccomplish surface crosslinking of the particulate polymer. The surfacecrosslinked particulate material was then post treated with the SurfaceTreatment set forth in Table 1.

TABLE 1 Comparative Examples 1 & 2 and Examples 1-7 Free swell CRC (0psi) AUL Surface Treatments¹ g/g GBP, Darcy (0.9 psi) Comparative 5 wt %of a 1.6 wt % aqueous solution of 34.7 3 16.4 Example 1 PEG 8000Comparative 5 wt % water and then 0.5 wt % Sipernat ® 34.8 7 14.4Example 2 22S silica Example 1 5.5 wt % of a 9.09 wt % AlPO₄ slurry 35.119 17.8 (prepared from 2.5 parts AlPO₄ press cake and 3 parts water) todeliver 0.5% AlPO₄ and 5% water Example 2 5.25 wt % of a 4.76 wt % AlPO₄slurry 34.8 19 18.4 (prepared from 1.25 parts AlPO₄ press cake and 4.25parts water) to deliver 0.25 wt % AlPO₄ and 5 wt % water Example 3 5.025wt % of a 0.498 wt % AlPO₄ 35.5 17 16 slurry (prepared from 0.125 partsAlPO₄ press cake, 2.5 parts 10% aqueous Lupamin ® 9095, and 2.4 partswater) to deliver 0.025 wt % AlPO₄, 0.25 wt % Lupamin ® 9095, and 4.75wt % water Example 4 5.25 wt % of a 4.76 wt % AlPO₄ slurry 34.8 22 17.3(prepared from 1.25 parts AlPO₄ press cake, 2.5 parts 10% aqueousLupamin ® 9095, and 1.75 parts water) to deliver 0.25 wt % AlPO₄, 0.25wt % Lupamin ® 9095, and 4.75 wt % water Example 5 5.5 wt % 9.09 wt %TiBO slurry 35.1 24 16 (prepared from 2.52 parts TiBO press cake and2.97 parts water) to deliver 0.5% TiBO and 5% water Example 6 5.25 wt %of a 4.76 wt % TiBO slurry 35.1 19 18 (prepared from 1.26 parts TiBOpress cake and 4.24 parts water) to deliver 0.25 wt % TiBO and 5 wt %water Example 7 5.025 wt % of a 0.498 wt % TiBO slurry 35 13 15.5(prepared from 0.126 parts TiBO press cake, 2.5 parts 10% aqueousLupamin ® 9095, and 2.4 parts water) to deliver 0.025 wt % TiBO, 0.25 wt% Lupamin ® 9095, and 4.75 wt % water ¹Post-treatment component added incombination with 5% wt water of preproduct. 2. Lupamin ® 9095 aqueoussolution of polyvinyl amine

Example 8

A blend of 2.5 parts of AlPO presscake, 1 part ethylene carbonate and 2part water was prepared. The blend is sprayed onto 100 parts ofPreproduct A with an air atomizer nozzle. The coated Preproduct A wasfed at a rate of 60-70 grams/minute into a continuous paddle reactorwith a peak temperature of 215° C. and a residence time of about 50minutes to accomplish surface crosslinking of the particulate polymer.The surface crosslinked particulate material was then post treated with2 wt % of a 10 wt % LUPAMIN® 9095 aqueous solution. The properties ofExample 8 are CRC of 34.38 g/g; Free swell (0 psi) GBP, 51.0 Darcy; andAUL(0.9 psi) of 16.2.

Preproduct B

In an insulated, flat-bottomed reaction vessel, 1866.7 g of 50 wt % NaOHwas added to 3090.26 g of distilled water and cooled to 25° C. 800 g ofacrylic acid was then added to the caustic solution and the solutionagain cooled to 25° C. A second solution of 1600 g of acrylic acidcontaining 120 g of 50% by weight methoxypolyethyleneglycolmonomethacrylate in acrylic acid and 14.4 g of ethoxylatedtrimethylolpropanetriacrylate was then added to the first solution,followed by cooling to 5° C., all while stirring. The monomer solutionwas then polymerized with a mixture of 100 ppm hydrogen peroxide, 200ppm azo-bis-(2-amidino-propene)dihydrochloride, 200 ppm sodiumpersulfate, and 40 ppm ascorbic acid (all aqueous solutions) underadiabatic conditions and held near the maximum temperature (T_(max)) for25 minutes. The resulting gel was chopped and extruded with a Hobart 4M6commercial extruder, followed by drying in a Procter & Schwartz Model062 forced air oven at 175° C. for 10 minutes with up flow and 6 minuteswith down flow air on a 20 in×40 in perforated metal tray to a finalproduct moisture level of less than 5 wt %. The dried material wascoarse-ground in a Prodeva Model 315-S crusher, milled in an MPI 666-Fthree-stage roller mill and sieved with an Minox MTS 600DS3V to removeparticles greater than 850 μm and smaller than 150 μm.

Example 9

Preproduct B was coated in an Anvil MIX9180 mixer with 1% ethylenecarbonate, 4% water, 0.5% Preproduct AlPO₄ Slurry as described above,350 ppm Chemcor 43G40SP (available from Chemcor Corporation, ChesterN.Y.) maleated polypropylene, and 0.2% SIPERNAT® 22S silica based on thedry superabsorbent polymer composition weight. The coated superabsorbentpolymer was heat treated to about 205° C. for about 45-50 minutesresidence time in order to effectuate the surface crosslinking of thepolymer particles.

After surface crosslinking the resulting particles were cooled to roomtemperature and then were post-treated by spraying the particles with a2 wt % of a solution prepared from parts of LUPAMIN® 9095 polyvinylamine solution and 95 parts water in a kitchen type mixer with a wirewhisk. The resultant product was allowed to equilibrate for at least 2hours, and then sieved through U.S. standard #20 mesh screen andretained on U.S. standard #100 mesh screen.

TABLE 2 Example 9 Free swell Metal Polymeric CRC (0 psi) ExampleCompound Coating g/g GBP, Darcy 9 0.5% AlPO₄ 350 ppm MPP² 33.1 42.13 2wt % of 5 wt % LUPAMIN 9095¹ ¹LUPAMIN ® 9095polyvinyl amine solution²Meleated Polypropylene

In addition, the foregoing example was sieved using a RO-TAP® MechanicalSieve Shaker Model B available from W. S. Tyler, Inc., Mentor Ohio andfound to have the following particle size distribution as set forth inTable 3.

TABLE 3 Example 9 Particle Size Distribution Particle Size Average % %on 20 mesh (>850 μm) 0.22 % on 30 mesh (600-850 μm) 20.79 % on 50 mesh(300-600 μm) 63.35 % on 170 mesh (45-90 μm) 15.6 % on 325 mesh (45-90μm) 0.04 % on 325 mesh (<45 μm) 0

Example 10

Preproduct B was continuously coated in a Shugi mixer with 3% by weightof preproduct of a 33% aqueous EC solution, 2.5% by weight of preproductof a 20% aluminum phosphate slurry, and 0.2% by weight of preproduct ofSIPERNAT® S22S. The aluminum phosphate slurry was blended with Chemcor43G40SP meleated polypropylene (MPP) to deliver 350 ppm MPP in thealuminum phosphate spray. Coated Preproduct B was then fed into acontinuous paddle reactor for a residence time of about 30 minutes and apeak superabsorbent temperature of about 199° C. Surface-crosslinkedsuperabsorbent polymer particles were then cooled and post-treated with2% of a solution prepared from 5 parts LUPAMIN® polyvinyl amine solutionand 95 parts water in an Anvil Model Number MIX9180 kitchen type mixerwith a wire whisk. After post-treatment, superabsorbent polymercomposition particles were allowed to stand for at least 2 hours priorto sieving through a U.S. standard #20 mesh and retained on U.S.standard #100 mesh screen. Example 10 superabsorbent polymer compositionparticles properties appear in Table 4.

TABLE 4 Example 10 CRC 0.0 GBP, AUL Example g/g Darcy (0.9 psi) 10 33.247 16.6

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients, reaction conditions, andso forth used in the specification and claims are to be understood asbeing modified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

What is claimed:
 1. A high-capacity superabsorbent polymer compositioncomprising a superabsorbent polymer comprising: a) from about 55% toabout 99.9% by weight of polymerizable unsaturated acid group containingmonomer, based on the superabsorbent polymer; and b) from about 0.001%to about 5% by weight of internal crosslinking agent based on thepolymerizable unsaturated acid group containing monomer; wherein thesuperabsorbent polymer has a degree of neutralization of greater thanabout 25%; wherein elements a) and b) are polymerized and prepared intosuperabsorbent polymer particles and further comprising the followingsurface additives to form surface treated superabsorbent polymerparticles treated with i) from about 0.01 to about 2% by weight of aninorganic metal compound, wherein said inorganic metal compound isselected from calcium phosphate, aluminum phosphate, or iron phosphate,based on the dry superabsorbent polymer composition, and ii) from about0.01% to about 0.5% by weight of cationic polymer based on thesuperabsorbent polymer composition and iii) from about 0.01% to about0.5% of a thermoplastic polymer that is selected from polyolefin,polyethylene, linear low density polyethylene, ethylene acrylic acidcopolymer, styrene copolymers, ethylene alkyl methacrylate copolymer,polypropylene, maleated polypropylene, ethylene vinyl acetate copolymer,polyamide, polyester, blends thereof, or copolymers thereof wherein thesuperabsorbent polymer composition exhibits a free swell gel bedpermeability of from 10 Darcy to about 47 Darcy as measured by the FreeSwell Gel Bed Permeability Test.
 2. The high-capacity superabsorbentpolymer composition according to claim 1 wherein the particles ofinorganic metal compound have a median particle size of less than 2 μm.3. The high-capacity superabsorbent polymer composition of claim 1having a centrifuge retention capacity of at least about 32 g/g asmeasured by the Centrifuge Retention Capacity Test.
 4. The high-capacitysuperabsorbent polymer composition of claim 1 wherein at least about 40%by weight of the dry superabsorbent polymer particles surface treatedhave a particle size from about 300 μm to about 600 μm.
 5. Thehigh-capacity superabsorbent polymer composition according to claim 1wherein the particles of the water-insoluble inorganic metal compoundhave a mass median particle size of less than about 1 μm.
 6. Thehigh-capacity superabsorbent polymer composition according to claim 1wherein the surface additives further comprise from about 0.01 wt % ofthe dry superabsorbent polymer composition of a silica.
 7. Thehigh-capacity superabsorbent polymer composition according to claim 1wherein the surface additives further comprise an anti-caking additive.8. The high-capacity superabsorbent polymer composition according toclaim 1 wherein the surface additives further comprise a flowmodification agent.
 9. The high-capacity superabsorbent polymercomposition according to claim 1 wherein the surface additives furthercomprise an odor-binding substance.
 10. The high-capacity superabsorbentpolymer composition according to claim 1 wherein the surface additivesfurther comprise a viscosity modifier.